This invention relates to implementing a low voltage start-up function for an integrated circuit (IC). A preferred embodiment of the invention is described for a direct current to direct current (DC-DC) conversion circuit with a low input voltage and an output load. However, persons skilled in the art will appreciate that the present invention can be applied to other ICs as well.
The present invention enables the internal voltage of the circuit to be increased without the negative effect of an external load draining that voltage before it can build to a sustainable level. The increased internal voltage also serves to reduce the on-resistance of the IC. The invention also enables this low voltage start-up function without adding a dedicated external control pin.
Step-up DC-DC conversion circuits can be implemented in an inductor based topology or in a switched capacitor based topology (e.g., a charge pump). The inductor based topology is generally preferred for low input voltage applications because it has greater package efficiency and start-up capability than the capacitor based topology. For example, a conversion circuit that boosts the input voltage four times (a quadrupler) can be implemented in the inductor based topology with a minimum of three pins while the capacitor based topology requires at least eight pins. The inductor based topology also requires less power switches to implement a quadrupler than the capacitor based topology. Thus, the inductor based topology has a lower on-resistance since it has less power switches and, therefore, it provides advantages for low voltage applications.
The inductor based topology, however, also has some disadvantages compared to the capacitor based topology. For instance, an inductor based topology is more complex to design than a capacitor based topology because the former requires numerous external components in addition to the internal IC controls. Moreover, inductor based topologies require magnetic energy storage which is difficult to stabilize and radiates EMI waves.
Inductor and capacitor based topologies do not provide adequate switch drive capability necessary for the low on-resistance required for an IC to operate at low input voltages. For instance, a quadrupler charge pump based on the capacitor topology requires a minimum of eight power switches and eight external pins. Such a capacitor based topology is difficult to start-up at low input voltages because of the high on-resistance of the power switches. Generally, capacitor based topology charge pumps have a higher on-resistance than inductor based topology charge pumps because of the extra power switches necessary to implement a charge pump based on a capacitor topology (eight switches versus one switch in the quadrupler discussed above). The resistive drops in the power switches caused by the inadequate drive voltage are effectively multiplied in the capacitor based topology by the additional power switches.
The addition of an external load creates another issue for the IC because the internal voltage needed to drive the power switches is drained across the external load before it can build to a sustainable level. Therefore, it is desirable to provide a switched capacitor based DC-DC converter that can enable low voltage start-up, operate at very low input voltages, and will not require extra control pins.
The present invention can be applied to any IC where the internal bias voltage is drained across an external load during start-up. The load drain in these circuits exacerbates the inefficiencies caused by the on-resistance of the circuit.